1. Field of the Invention
The invention relates to an exhaust gas control apparatus of an internal combustion engine, that includes a catalyst that is provided in an exhaust passage of an internal combustion engine and has the ability to store oxygen, and an oxygen sensor that is provided on the exhaust gas downstream side of the catalyst in the exhaust passage and outputs a signal indicative of an air-fuel ratio of exhaust gas. The invention also relates to a method for determining an abnormality in this exhaust gas control apparatus.
2. Description of the Related Art
An exhaust gas control apparatus of an internal combustion engine purifies carbon monoxide (CO), hydrocarbons (HC), and oxides of nitrogen (NOx) in exhaust gas by oxidizing the CO and HC and reducing the NOx using a catalyst provided in an exhaust passage. Here, when the catalyst atmosphere is a stoichiometric air-fuel ratio, the purifying reactions (i.e., the oxidation-reduction reactions) of the HC, CO, and NOx can be performed simultaneously. However, when the catalyst atmosphere is different than the stoichiometric air-fuel ratio, the purifying reactions of the HC, CO, and NOx are not able to be performed simultaneously. Therefore, as described in Japanese Patent Application Publication No. 2007-154749 (JP-A-2007-154749), for example, a decrease in exhaust gas purifying efficiency due to deviation of the air-fuel ratio such as that described above is suppressed by compensating for the excess or deficiency of oxygen that occurs due to the temporary deviation of the air-fuel ratio, by providing a catalyst capable of storing oxygen in the exhaust passage.
In such an exhaust gas control catalyst, an air-fuel ratio sensor that outputs a signal proportionate to the air-fuel ratio of the exhaust gas is provided on the exhaust gas upstream side of the catalyst in the exhaust passage. Also, an oxygen sensor that outputs a signal indicative of the air-fuel ratio of the exhaust gas, or more specifically, that outputs approximately 0 V when the air-fuel ratio of the exhaust gas is leaner than the stoichiometric air-fuel ratio and outputs approximately 1 V when the air-fuel ratio of the exhaust gas is richer than the stoichiometric air-fuel ratio, is provided on the exhaust gas downstream side of the catalyst.
Incidentally, the ability of the catalyst to store oxygen (i.e., the oxygen storage capacity) decreases due to degradation and the like of the catalyst. Therefore, the ability of the catalyst to store oxygen, i.e., the degree of degradation of the catalyst, is ascertained by estimating the maximum oxygen storage amount of the catalyst. More specifically, the catalyst is made to release oxygen by forcibly making the air-fuel ratio of the exhaust gas that flows into the catalyst richer than the stoichiometric air-fuel ratio. Then when the catalyst is no longer able to release any more oxygen, the output of the oxygen sensor will make a rich reversal from 0 V to 1 V, i.e., will reverse to rich. When the output of the oxygen sensor makes a rich reversal, the catalyst is made to store oxygen by forcibly making the air-fuel ratio of the exhaust gas that flows into the catalyst leaner than the stoichiometric air-fuel ratio. Then when no more oxygen is able to be stored in the catalyst, the output of the oxygen sensor will make a lean reversal from 1 V to 0 V, i.e., will reverse to lean. Here, the maximum oxygen storage amount of the catalyst corresponds to the amount of oxygen that flows into the catalyst during a period of time from when the output of the oxygen sensor makes a rich reversal until the output of the oxygen sensor makes a lean reversal. Therefore, the amount of oxygen that flows into the catalyst per unit of time can be estimated based on the operating state of the engine, and the maximum oxygen storage amount can be estimated by integrating this oxygen amount over this period of time. Also, the maximum oxygen release amount of the catalyst corresponds to the amount of oxygen released from the catalyst during a period of time from when the output of the oxygen sensor makes a lean reversal until the output of the oxygen sensor makes a rich reversal. Therefore, the amount of oxygen released from the catalyst per unit of time can be estimated based on the operating state of the engine, and the maximum oxygen release amount can be estimated by integrating this oxygen amount over this period of time.
Incidentally, a response delay in the rich reversal or the lean reversal of the output of the oxygen sensor may occur for some reason. In this case, the output of the oxygen sensor makes a rich reversal later than the timing at which the actual air-fuel ratio of the exhaust gas near the oxygen sensor reverses from leaner than the stoichiometric air-fuel ratio to richer than the stoichiometric air-fuel ratio. Also, the output of the oxygen sensor makes a lean reversal later than the timing at which the actual air-fuel ratio of the exhaust gas near the oxygen sensor reverses from richer than the stoichiometric air-fuel ratio to leaner than the stoichiometric air-fuel ratio. Therefore, the maximum oxygen storage amount or the maximum oxygen release amount is no longer able to be accurately estimated. As a result, when making a determination as to whether there is an abnormality in the catalyst based on the maximum oxygen storage amount or the maximum oxygen release amount, this determination is not able to be made accurately.